The medial superior olive (MSO) is presumed to play a major role in sound localization. It is the initial site in the auditory CNS where sound-evoked activity from both ears converges on and excites individual neurons. It has been proposed that cells located here can, by comparing these binaural inputs, extract and respond to important temporal cues available in this incoming signal. These temporal cues change as a sound changes location in azimuthal space and it has also been proposed that different members of the MSO cell population respond optimally to only those binaural cues generated from a particular point(s) in this space. Thus, important aspects of the "map" of external space are extracted and neurally encoded in a population of selectively sensitive neurons. Unfortunately, the great difficulty in recording from positively identified MSO cells, using standard in vivo extracellular and intracellular methods, has hindered verification of such hypotheses. We intend to do both in vitro intracellular recordings from MSO cells (and from cells in periolivary nuclei inputting to MSO) and in vivo intraaxonal recordings from MSO axons using high impedance Neurobiotin-filled glass electrodes. In vitro, we will characterize intrinsic membrane features of - and synaptic inputs to - MSO cells to distinguish aspects that could be important in accomplishing the localization tasks described above. We will then study the anatomical aspects of the labeled cell to determine local circuitry and if certain cell subtypes show unique physiology. In vivo, we will record intraaxonally from axons of MSO cells, as they approach inferior colliculus in lateral lemniscus, and characterize their responses to monaural and binaural stimuli. We will subsequently analyze 1) the labeled axon, at light and electron microscopic levels, to determine the influence of MSO cells on other auditory nuclei and 2) the location and type of the backfilled MSO cell body to determine 1) cell type 2) its synaptic input 3) whether different cell types encode different auditory features and 4) if they are arranged in any sort of order (map) based on such features (e.g. those features used to encode location in space).